You need to get Rank A before any interaction or final event stage for the final chapter for any heroine route for a True Ending ( One of the link above stated you need Rank S, which is not true because I personally confirm this with Rank B and Rank A on several route.
( You get the True Ending if you have the other requirement listed below)
51% (or higher) Affection will get you Good Ending route but can change to True Ending based on what you do during the final chapter and get 100% in the end.
Here some quick Facts (from my personal gameplay experiences) like a per-requirement for True Ending: I got all the true ending trophy but one so I have look it up and while I'm at it might as well write how to get it in a simple form. Another reason is that I purposely have look for these guide to find the final True Ending for myself. I will tell you why the reason is that I don't have to repeat what already stated on those link and only mention what not mention on those links. Now you maybe asking why bother writing some more if you're going put up some links. Gal*Gun: Double Peace Trophy Guide and Road MapĮasy to use Guide to platinum Gal*Gun Double peace
Here are some links that quite helpful in telling you how: